1. Field of the Invention
The present invention relates to activated carbon and methods for preparing the same. Particularly, the invention relates to an improved process for preparing activated carbon with a high surface area by swelling a carbon-containing material in a polar solvent containing oxidizing agents prior to carbonization.
2. Description of the Prior Art
Activated carbon is a microcrystalline, nongraphitic form of carbon which has been processed to increase its porosity. Activated carbon is characterized by a large specific surface area, typically in the range of 800 to 1500 m.sup.2 per gram, and in some cases as high as 2500 m.sup.2 per gram.
Due to its large surface area, activated carbons have a great variety of applications, both as catalyst supports and adsorbents. As catalyst supports, activated carbons provide large surface areas for chemical reactions and are more stable at high temperatures than common inorganic sieves.
The adsorbent properties of activated carbon is the result of its large surface area, high degree of surface reactivity, and favorable pore size that make the internal surface area accessible to gases and liquids. (Bansal et al., Activated CARBON, Marcel Dekker, Inc., New York, pp 1-26 (1988)). Generally, the larger the surface area of the activated carbon, the greater is its adsorption capacity. The available surface area of activated carbon is dependent upon its pore volume. Since the surface area per unit volume decreases as individual pore size increases, a large surface area is optimized by maximizing the number of pores of very small dimensions and/or minimizing the number of pores of very large dimensions.
As adsorbents, activated carbons can be used in environmentally-related processes to remove pollutants from liquid and gaseous streams very efficiently. Activated carbons may also be used for gaseous storage materials. One of the most promising applications is for methane storage. The adsorption of methane in the micropores of activated carbon increases the density of the gas. Thus, the amount of methane that can be stored using the adsorbent is much higher than can be obtained under pressure.
Different kinds of raw materials have been made into activated carbons, including cellulosic materials of various plant origins, peat, lignite, soft and hard coals, tars and pitches, asphalt, petroleum residues, carbon black, and polymers such as poly(vinylidenechloride), Saran, and poly(vinylchloride). Coal, particularly the low rank type, has been found to be a good raw material for the production of activated carbons.
The preparation of activated carbons generally involves two steps. The first step is carbonization of the carbonaceous raw material at temperatures below 800.degree. C. in the absence of oxygen, and the second step is activation of the carbonized product. (Bansal et al., Activated CARBON, Marcel Dekker, Inc., New York, pp 1-26 (1988)).
During the first step of carbonization, most of the noncarbon elements such as oxygen and hydrogen are eliminated as volatile gases by pyrolytic decomposition of the starting material. The residual carbon material forms an irregular structure, leaving free interstices that may be filled or blocked by disorganized carbon resulting from deposition of tars. Thus, because the pores of the product are not fully developed and therefore the surface area is low, carbonization does not give rise to products that have high adsorption capacity.
Carbonization involves two important physical changes in the raw material that markedly determine the properties of the final product. The first change is a thermal softening of the material, induced by heating. During this change, it is important to control the temperature, as the temperature and its rate of change affect the type of char obtained. After the thermal softening period, the char undergoes a second change as it begins to harden and shrink. This shrinkage also plays a role in the development of porosity in the char. For example, when soft coal is used as the starting material, the temperature rise during the softening stage should be very slow, so that the gases can escape through the pores in the granules without a collapse or deformation of the pores.
Activation, the second step of producing activated carbon, enhances the rudimentary pore structure formed during carbonization. During this step, the diameter of the pores is enlarged and the volume of the pores is increased. Activation is carried out by an oxidation reaction at high temperatures. The oxidizing agents are usually steam, carbon dioxide, air, or a mixture of these. During this physical activation, the char is partially gasified, thus resulting in some loss of the carbonized material. However, oxidation converts the carbonized raw material into a form that contains the greatest possible number of randomly distributed pores of various shapes and sizes, and a final product with a high surface area.
Carbonization and activation are sometimes carried out simultaneously, by thermal decomposition of the raw material impregnated with chemical activating agents. These activating chemicals are both dehydrating agents and oxidants. The dehydrating activity inhibits the formation of tar, thus enhancing the yield of carbon in the final product. For example, carbonization may be carried out in the presence of chemical reagents such as phosphoric acid, potassium hydroxide, calcium chloride, or zinc chloride.
The most common method of simultaneous carbonization and oxidation is to impregnate the raw material with an activating agent. This is accomplished by soaking the raw material in a concentrated aqueous solution of the reagent, sometimes accompanied by mixing and kneading. The water is then evaporated from the chemically impregnated material, and the resulting mixture is carbonized in the absence of air (such as under nitrogen) at temperatures between 400.degree. and 800 .degree. C., depending upon the activating agent used. These temperatures are lower than those needed for "physical activation," and result in better porous structural development. The carbonized material is then washed to remove the chemical reagent, which may then be recovered and recycled. The surface area of this material is usually further increased by gasification. This step also results in further loss of carbonized material.
One early method and apparatus for the manufacture of activated charcoal in which the carbonizing and activating steps were combined in a single unit was provided by Shirai et al., U.S. Pat. No. 3,676,365. The disclosed process allowed continuous performance of the carbonization and activation steps at high efficiency. In this method, production of activated carbon occurred by impregnation of a carbon source with a solution of ZnCl.sub.2 and hydrochloric acid.
Several other methods of chemically activating a carbon source prior to pyrolysis to produce activated carbon are described by Zhiquan Q. Yan, U.S. Pat. No. 5,250,491; Caio et al., Revista Brasileira de Tecnologia, 10:93-107 (1979); East German Patent No. 146 277; European Patent Application No. 0 329 251; and Caturla et al., Carbon 29: 999-1007 (1991). These methods of combining carbonization with chemical activation suffer from several limitations. Gasification following the carbonization step usually results in a loss of about 25% of the activated carbon. Diffusion of the activating agent into the raw material is limited by the degree of penetration of the activating agent into the innermost part of the coal particles.
An efficient method of producing activated charcoal of high surface area and narrow pore size distribution is needed.